BIOCHEMISTRY A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site vitamin The structure of some vitamins. Closely related compounds often have similar metabolic effects, thus vitamin B6 exists in aldehyde and amine forms as well as the alcohol shown here. An organic molecule needed in trace amounts for normal growth and metabolic processes. Vitamins often act as part of an enzyme system (hence the small amounts required), and in doing so, are broken down and lost. What distinguishes a vitamin from most other compounds essential in metabolism is that the organism can't replace it by synthesizing it, and so it must get it from outside. Occasionally an animal may be able to synthesize part of its requirement (e.g. vitamin D in humans), and often symbiotic organisms provide part or all of the requirement (e.g. B vitamins synthesized by gut bacteria of insects and vertebrates). Vitamins are needed only by heterotrophic organisms; autotrophs, such as most green plants, are by definition independent of an external supply of organic compounds. What is a vitamin for one heterotroph may be synthesized in adequate amounts by, and is therefore not a vitamin for, another; or it may take no part at all in the metabolism of another. There is no such thing as a vitamin in general, but only for specified organisms. Some (like those of the vitamin B complex) which are perhaps universal constituents of existing organisms are however required by a very wide range of organism; others (like C) by very few. Every vitamin necessary for a given organism is synthesized by other organisms, otherwise a continuous supply wouldn't be available. Sometimes several different compounds can substitute for each other in satisfying a given requirement; either because the organism requires, not a specific molecule, but a specific chemical grouping which is present in, and available from, each of the alternative compounds; or because conversion of a few closely related groupings into the one required is possible within the organism. Deficiency of the vitamin reduces the rate of the metabolic process in which it is concerned, with widespread effects (symptoms of deficiency disease). A general effect of deficiency of most vitamins, which was important in the early history of their discovery, is that growth of young animals is stunted. Related category    • BIOCHEMISTRY Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP